Method and system for doppler detection and doppler correction of optical chirped range detection

ABSTRACT

Techniques for Doppler correction of chirped optical range detection include obtaining a first set of ranges based on corresponding frequency differences between a return optical signal and a first chirped transmitted optical signal with an up chirp that increases frequency with time. A second set of ranges is obtained based on corresponding frequency differences between a return optical signal and a second chirped transmitted optical signal with a down chirp. A matrix of values for a cost function is determined, one value for each pair of ranges that includes one in the first set and one in the second set. A matched pair of one range in the first set and a corresponding one range in the second set is determined based on the matrix. A Doppler effect on range is determined based on combining the matched pair of ranges. A device is operated based on the Doppler effect.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of Patent Cooperation Treaty (PCT)Appln. PCT/US2017/62703 Filed Nov. 21, 2017, which claims priority toProvisional Appln. 62/428,109, filed Nov. 30, 2016, the entire contentsof which are hereby incorporated by reference as if fully set forthherein, under 35 U.S.C. § 119(e).

STATEMENT OF GOVERNMENTAL INTEREST

This invention was made with government support under contractW9132V-14-C-0002 awarded by the Department of the Army. The governmenthas certain rights in the invention.

BACKGROUND

Optical detection of range, often referenced by a mnemonic, LIDAR, forlight detection and ranging, is used for a variety of applications, fromaltimetry, to imaging, to collision avoidance. LIDAR provides finerscale range resolution with smaller beam sizes than conventionalmicrowave ranging systems, such as radio-wave detection and ranging(RADAR). Optical detection of range can be accomplished with severaldifferent techniques, including direct ranging based on round triptravel time of an optical pulse to a target, and chirped detection basedon a frequency difference between a transmitted chirped optical signaland a returned signal scattered from a target.

To achieve acceptable range accuracy and detection sensitivity, directlong range LIDAR systems use short pulse lasers with low pulserepetition rate and extremely high pulse peak power. The high pulsepower can lead to rapid degradation of optical components. Chirped LIDARsystems use long optical pulses with relatively low peak optical power.In this configuration, the range accuracy depends on the chirp bandwidthrather than the pulse duration, and therefore excellent range accuracycan still be obtained.

Useful optical chirp bandwidths have been achieved using wideband radiofrequency (RF) electrical signals to modulate an optical carrier. Recentadvances in chirped LIDAR include using the same modulated opticalcarrier as a reference signal that is combined with the returned signalat an optical detector to produce in the resulting electrical signal arelatively low beat frequency that is proportional to the difference infrequencies between the references and returned optical signals. Thiskind of beat frequency detection of frequency differences at a detectoris called heterodyne detection. It has several advantages known in theart, such as the advantage of using RF components of ready andinexpensive availability. Recent work described in U.S. Pat. No.7,742,152 shows a novel simpler arrangement of optical components thatuses, as the reference optical signal, an optical signal split from thetransmitted optical signal. This arrangement is called homodynedetection in that patent.

SUMMARY

The current inventors have recognized circumstances and applications inwhich motion of an object, to which range is being detected using anoptical chirp, noticeably affects such applications due to Dopplerfrequency shifts. Techniques are provided for detecting the Dopplereffect and compensating for the Doppler effect in such optical chirprange measurements.

In a first set of embodiments, a method implemented on a processorincludes obtaining a first set of one or more ranges based oncorresponding frequency differences in a return optical signal comparedto a first chirped transmitted optical signal. The first chirpedtransmitted optical signal includes an up chirp that increases itsfrequency with time. The method further includes obtaining a second setof one or more ranges based on corresponding frequency differences in areturn optical signal compared to a second chirped transmitted opticalsignal. The second chirped transmitted optical signal includes a downchirp that decreases its frequency with time. The method still furtherincludes determining a matrix of values for a cost function, one valuefor the cost function for each pair of ranges, in which each pair ofranges includes one range in the first set and one range in the secondset. Even further, the method includes determining a matched pair ofranges which includes one range in the first set and a corresponding onerange in the second set, where the correspondence is based on the matrixof values. Still further, the method includes determining a Dopplereffect on range based on combining the matched pair of ranges. Stillfurther, the method includes operating a device based on the Dopplereffect.

In a second set of embodiments, an apparatus includes a laser sourceconfigured to provide a first optical signal consisting of an up chirpin a first optical frequency band and a simultaneous down chirp in asecond optical frequency band that does not overlap the first opticalfrequency band. The apparatus includes a first splitter configured toreceive the first signal and produce a transmitted signal and areference signal. The apparatus also includes an optical couplerconfigured to direct the transmitted signal outside the apparatus and toreceive any return signal backscattered from any object illuminated bythe transmitted signal. The apparatus also incudes a frequency shifterconfigured to shift the transmitted signal or the return signal a knownfrequency shift relative to the reference signal. Still further, theapparatus includes an optical detector disposed to receive the referencesignal and the return signal after the known frequency shift is applied.In addition, the apparatus still further includes a processor configuredto perform the steps of receiving an electrical signal from the opticaldetector. The processor is further configured to support determinationof a Doppler effect due to motion of any object illuminated by thetransmitted signal by determining a first set of zero or more beatfrequencies in a first frequency band and a second set of zero or morebeat frequencies in a second non-overlapping frequency band of theelectrical signal. The first frequency band and the secondnon-overlapping frequency band are determined based on the knownfrequency shift.

In some embodiments of the second set, the laser source is made up of alaser, a radio frequency waveform generator, and a modulator. The laseris configured to provide a light beam with carrier frequency ƒ0. Theradio frequency waveform generator is configured to generate a firstchirp in a radio frequency band extending between ƒa and ƒb, whereinƒb>ƒa>0. The modulator is configured to produce, based on the firstchirp, the first optical signal in which the first optical frequencyband is in a first sideband of the carrier frequency and the secondoptical frequency band is in a second sideband that does not overlap thefirst sideband.

In a third set of embodiments, an apparatus includes a laser sourceconfigured to provide a first optical signal consisting of an up chirpin a first optical frequency band and a simultaneous down chirp in asecond optical frequency band that does not overlap the first opticalfrequency band. The apparatus includes a first splitter configured toreceive the first signal and produce a transmitted signal and areference signal. The apparatus also includes an optical couplerconfigured to direct the transmitted signal outside the apparatus and toreceive any return signal backscattered from any object illuminated bythe transmitted signal. Still further, the apparatus includes a secondsplitter configured to produce two copies of the reference signal andtwo copies of the return signal. Even further, the apparatus includestwo optical filters. A first optical filter is configured to pass thefirst optical frequency band and block the second optical frequencyband. A second optical filter is configured to pass the second opticalfrequency band and block the first optical sideband. The apparatus yetfurther includes two optical detectors. A first optical detector isdisposed to receive one copy of the reference signal and one copy of thereturn signal after passing through the first optical filter. A secondoptical detector is disposed to receive a different copy of thereference signal and a different copy of the return signal after passingthrough the second optical filter. In addition, the apparatus stillfurther includes a processor configured to perform the steps ofreceiving a first electrical signal from the first optical detector anda second electrical signal from the second optical detector. Theprocessor is further configured to support determination of a Dopplereffect due to motion of any object illuminated by the transmitted signalby determining a first set of zero or more beat frequencies in the firstelectrical signal, and determining a second set of zero or more beatfrequencies in the second electrical signal.

In other embodiments, a system or apparatus or computer-readable mediumis configured to perform one or more steps of the above methods.

Still other aspects, features, and advantages are readily apparent fromthe following detailed description, simply by illustrating a number ofparticular embodiments and implementations, including the best modecontemplated for carrying out the invention. Other embodiments are alsocapable of other and different features and advantages, and its severaldetails can be modified in various obvious respects, all withoutdeparting from the spirit and scope of the invention. Accordingly, thedrawings and description are to be regarded as illustrative in nature,and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example, and not by way oflimitation, in the figures of the accompanying drawings in which likereference numerals refer to similar elements and in which:

FIG. 1A is a set of graphs that illustrates an example optical chirpmeasurement of range, according to an embodiment;

FIG. 1B is a graph that illustrates an example measurement of a beatfrequency resulting from de-chirping, which indicates range, accordingto an embodiment;

FIG. 2A and FIG. 2B are block diagrams that illustrate examplecomponents of a high resolution LIDAR system, according to variousembodiments;

FIG. 3A is a block diagram that illustrates example components of aheterodyne chirped LIDAR system, according to an embodiment;

FIG. 3B is a block diagram that illustrates example components of ahomodyne chirped LIDAR system, according to an embodiment;

FIG. 4A is a block diagram that illustrates an example graph showingranges, based on beat frequency peaks from an up chirp LIDAR system, toseveral targets when the targets are moving, according to an embodiment;

FIG. 4B is a block diagram that illustrates an example graph showingranges, based on beat frequency peaks from a down chirp LIDAR system, toseveral targets when the targets are moving, according to an embodiment;

FIG. 5A is a graph that illustrates an example serial up and down chirptransmitted optical signal for a LIDAR system, according to anembodiment;

FIG. 5B is a graph that illustrates an example simultaneous up and downchirp transmitted optical signal for a LIDAR system, according to anembodiment;

FIG. 5C is a graph that illustrates example first order sidebandsproduced by a modulator to generate simultaneous up and down chirptransmitted optical signal for a LIDAR system, according to anembodiment;

FIG. 5D is a graph that illustrates an example simultaneous up and downchirp transmitted optical signal and reference signal (LO) for a LIDARsystem relative to an optical carrier, according to another embodiment;

FIG. 5E is a graph that illustrates an example shifted simultaneous upand down chirp transmitted optical signal and reference signal (LO) fora LIDAR system relative to an optical carrier, according to anotherembodiment;

FIG. 6 is a block diagram that illustrates example components for asimultaneous up and down chirp LIDAR system, according to an embodiment;

FIG. 7A is a block diagram that illustrates example pairs of up chirpand down-chirp ranges, according to an embodiment;

FIG. 7B is a block diagram that illustrates an example cost matrix withelements corresponding to the example pairs of up chirp and down-chirpranges depicted in FIG. 7A, according to an embodiment;

FIG. 8 is a flow chart that illustrates an example method for using anup and down chirp LIDAR system to compensate for Doppler effects onranges, according to an embodiment;

FIG. 9A through 9C are images that illustrates an example output on adisplay device based on Doppler corrected ranges and improvement overnon-corrected ranges, according to embodiments;

FIG. 10A is a set of three images that illustrate an example output on adisplay device based on Doppler corrected ranges for a targetapproaching a LIDAR system, according to an embodiment;

FIG. 10B is a set of three images that illustrate an example output on adisplay device based on Doppler corrected ranges for a target retreatingfrom a LIDAR system, according to an embodiment;

FIG. 11 is an image that illustrates an example output on a displaydevice based on Doppler corrected ranges at high resolution, accordingto an embodiment;

FIG. 12 is a block diagram that illustrates a computer system upon whichan embodiment of the invention may be implemented; and

FIG. 13 illustrates a chip set upon which an embodiment of the inventionmay be implemented.

DETAILED DESCRIPTION

A method and apparatus and system and computer-readable medium aredescribed for Doppler correction of optical chirped range detection. Inthe following description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. It will be apparent, however, toone skilled in the art that the present invention may be practicedwithout these specific details. In other instances, well-knownstructures and devices are shown in block diagram form in order to avoidunnecessarily obscuring the present invention.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope are approximations, the numerical values set forth inspecific non-limiting examples are reported as precisely as possible.Any numerical value, however, inherently contains certain errorsnecessarily resulting from the standard deviation found in theirrespective testing measurements at the time of this writing.Furthermore, unless otherwise clear from the context, a numerical valuepresented herein has an implied precision given by the least significantdigit. Thus a value 1.1 implies a value from 1.05 to 1.15. The term“about” is used to indicate a broader range centered on the given value,and unless otherwise clear from the context implies a broader rangaround the least significant digit, such as “about 1.1” implies a rangefrom 1.0 to 1.2. If the least significant digit is unclear, then theterm “about” implies a factor of two, e.g., “about X” implies a value inthe range from 0.5X to 2X, for example, about 100 implies a value in arange from 50 to 200. Moreover, all ranges disclosed herein are to beunderstood to encompass any and all sub-ranges subsumed therein. Forexample, a range of “less than 10” can include any and all sub-rangesbetween (and including) the minimum value of zero and the maximum valueof 10, that is, any and all sub-ranges having a minimum value of equalto or greater than zero and a maximum value of equal to or less than 10,e.g., 1 to 4.

Some embodiments of the invention are described below in the context ofa linear frequency modulated optical signal but chirps need not belinear and can vary frequency according to any time varying rate ofchange. Embodiments are described in the context of a single opticalbeam and its return on a single detector or pair of detectors, which canthen be scanned using any known scanning means, such as linear steppingor rotating optical components or with arrays of transmitters anddetectors or pairs of detectors.

1. Chirped Detection Overview

FIG. 1A is a set of graphs 110, 120, 130, 140 that illustrates anexample optical chirp measurement of range, according to an embodiment.The horizontal axis 112 is the same for all four graphs and indicatestime in arbitrary units, on the order of milliseconds (ms, 1 ms=10⁻³seconds). Graph 110 indicates the power of a beam of light used as atransmitted optical signal. The vertical axis 114 in graph 110 indicatespower of the transmitted signal in arbitrary units. Trace 116 indicatesthat the power is on for a limited pulse duration, r starting at time 0.Graph 120 indicates the frequency of the transmitted signal. Thevertical axis 124 indicates the frequency transmitted in arbitraryunits. The trace 126 indicates that the frequency of the pulse increasesfrom ƒ₁ to ƒ₂ over the duration τ of the pulse, and thus has a bandwidthB=ƒ₂−ƒ₁. The frequency rate of change is (ƒ₂−ƒ₁)/τ.

The returned signal is depicted in graph 130 which has a horizontal axis112 that indicates time and a vertical axis 124 that indicates frequencyas in graph 120. The chirp 126 of graph 120 is also plotted as a dottedline on graph 130. A first returned signal is given by trace 136 a,which is just the transmitted reference signal diminished in intensity(not shown) and delayed by Δt. When the returned signal is received froman external object after covering a distance of 2R, where R is the rangeto the target, the returned signal start at the delayed time Δt is givenby 2R/c, were c is the speed of light in the medium (approximately 3×10⁸meters per second, m/s). Over this time, the frequency has changed by anamount that depends on the range, called ƒ_(R), and given by thefrequency rate of change multiplied by the delay time. This is given byEquation 1a.ƒ_(R)=(ƒ₂−ƒ₁)/τ*2R/c=2BR/cτ  (1a)The value of ƒ_(R) is measured by the frequency difference between thetransmitted signal 126 and returned signal 136 a in a time domain mixingoperation referred to as de-chirping. So the range R is given byEquation 1b.R=ƒ _(R) cτ/2B  (1b)Of course, if the returned signal arrives after the pulse is completelytransmitted, that is, if 2R/c is greater than r, then Equations 1a and1b are not valid. In this case, the reference signal is delayed a knownor fixed amount to ensure the returned signal overlaps the referencesignal. The fixed or known delay time of the reference signal ismultiplied by the speed of light, c, to give an additional range that isadded to range computed from Equation 1b. While the absolute range maybe off due to uncertainty of the speed of light in the medium, this is anear-constant error and the relative ranges based on the frequencydifference are still very precise.

In some circumstances, a spot illuminated by the transmitted light beamencounters two or more different scatterers at different ranges, such asa front and a back of a semitransparent object, or the closer andfarther portions of an object at varying distances from the LIDAR, ortwo separate objects within the illuminated spot. In such circumstances,a second diminished intensity and differently delayed signal will alsobe received, indicated on graph 130 by trace 136 b. This will have adifferent measured value of ƒ_(R) that gives a different range usingEquation 1b. In some circumstances, multiple returned signals arereceived.

Graph 140 depicts the difference frequency ƒ_(R) between a firstreturned signal 136 a and the reference chirp 126. The horizontal axis112 indicates time as in all the other aligned graphs in FIG. 1A, andthe vertical axis 134 indicates frequency difference on a much expandedscale. Trace 146 depicts the constant frequency ƒ_(R) measured duringthe transmitted chirp, which indicates a particular range as given byEquation 1b. The second returned signal 136 b, if present, would giverise to a different, larger value of ƒ_(R) (not shown) duringde-chirping; and, as a consequence yield a larger range using Equation1b.

A common method for de-chirping is to direct both the reference opticalsignal and the returned optical signal to the same optical detector. Theelectrical output of the detector is dominated by a beat frequency thatis equal to, or otherwise depends on, the difference in the frequenciesof the two signals converging on the detector. A Fourier transform ofthis electrical output signal will yield a peak at the beat frequency.This beat frequency is in the radio frequency (RF) range of Megahertz(MHz, 1 MHz=10⁶ Hertz=10⁶ cycles per second) rather than in the opticalfrequency range of Terahertz (THz, 1 THz=10¹² Hertz). Such signals arereadily processed by common and inexpensive RF components, such as aFast Fourier Transform (FFT) algorithm running on a microprocessor or aspecially built FFT or other digital signal processing (DSP) integratedcircuit. In other embodiments, the return signal is mixed with acontinuous wave (CW) tone acting as the local oscillator (versus a chirpas the local oscillator). This leads to the detected signal which itselfis a chirp (or whatever waveform was transmitted). In this case thedetected signal would undergo matched filtering in the digital domain asdescribed in Kachelmyer 1990. The disadvantage is that the digitizerbandwidth requirement is generally higher. The positive aspects ofcoherent detection are otherwise retained.

FIG. 1B is a graph that illustrates an example measurement of a beatfrequency resulting from de-chirping, which indicates range, accordingto an embodiment. The horizontal axis 152 indicates frequency inMegahertz; and the vertical axis indicates returned signal power densityI_(R) relative to transmitted power density I_(T) in decibels (dB, Powerin dB=20 log(I_(R)/I_(T))). Trace 156 is the Fourier transform of theelectrical signal output by the optical detector, such as produced by aFFT circuit and is based on data published by Adany et al., 2009. Thehorizontal location of the peak gives ƒ_(R) and is used to estimate therange, using Equation 1b. In addition, other characteristics of the peakcan be used to describe the returned signal. For example, the powervalue at the peak is characterized by the maximum value of trace 156,or, more usually, by the difference 157 (about 19 dB in FIG. 1B) betweenthe peak value (about −31 dB in FIG. 1B) and a noise floor (about −50 dBin FIG. 1B) at the shoulders of the peak; and, the width of the peak ischaracterized by the frequency width 158 (about 0.08 MHz in FIG. 1B) athalf maximum (FWHM). If there are multiple discernable returns, therewill be multiple peaks in the FFT of the electrical output of theoptical detector, likely with multiple different power levels andwidths. Any method may be used to automatically identify peaks intraces, and characterize those peaks by location, height and width. Forexample, in some embodiments, FFTW or peak detection by MATLAB—SignalProcessing Toolbox is used, available from MATLAB™ of MATHWORKS™ ofNatick, Mass. One can also use custom implementations that rely on FFTWin CUDA and custom peak detection in CUDA™ available from NVIDIA™ ofSanta Clara, Calif. Custom implementations have been programmed on fieldprogrammable gate arrays (FPGAs). A commonly used algorithm is tothreshold the range profile and run a center of mass algorithm, peakfitting algorithm (3-point Gaussian fit), or nonlinear fit of the peakfor some function (such as a Gaussian) to determine the location of thepeak more precisely.

If the object detected (the source) is moving at velocity v_(s) and theLIDAR system (the observer) is moving at velocity v_(o) on the vectorconnecting the two, then the returned signal may be Doppler shifted andthe beat frequency detected ƒ_(R) is also shifted, which can lead toerrors in the detected range. In many circumstances, a shape of theobject being detected is identified based on the relative location ofmultiple returns. Thus the shape of the object may be in error and theability to identify the object may be compromised.

The observed frequency ƒ′ of the return differs from the correctfrequency ƒ of the return by the Doppler effect and is approximated byEquation 2a.

$\begin{matrix}{f^{\prime} = {\frac{\left( {c + v_{0}} \right)}{\left( {c + v_{s}} \right)}f}} & \left( {2a} \right)\end{matrix}$Where c is the speed of light in the medium. Note that the twofrequencies are the same if the observer and source are moving at thesame speed in the same direction on the vector between the two. Thedifference between the two frequencies, Δƒ=ƒ′−ƒ, is the Doppler shift,D, which constitutes an error in the range measurement, and is given byEquation 2b.

$\begin{matrix}{D = {\left\lbrack {\frac{\left( {c + v_{o}} \right)}{\left( {c + v_{s}} \right)} - 1} \right\rbrack f}} & \left( {2b} \right)\end{matrix}$Note that the magnitude of the error increases with the frequency ƒ ofthe signal. Note also that for a stationary LIDAR system (v_(o)=0), fora target moving at 10 meters a second (v_(o)=10), and visible light offrequency about 500 THz, then the size of the error is on the order of16 MHz which is 75% of the size of ƒ_(R), which is about 22 MHz in FIG.1B, leading to a 75% error in ƒ_(R) and thus a 75% error in range. Invarious embodiments, the Doppler shift error is detected and used tocorrect the range as described herein.

2. Chirped Detection Hardware Overview

In order to depict how the chirped detection approach is implemented,some generic and specific hardware approaches are described. FIG. 2A andFIG. 2B are block diagrams that illustrate example components of a highresolution LIDAR system, according to various embodiments. In FIG. 2A, alaser source 212 emits a carrier wave 201 that is frequency modulated inthe modulator 214 based on input from a RF waveform generator 215 toproduce a pulse that has a bandwidth B and a duration τ. In someembodiments, the RF waveform generator 215 is software controlled withcommands from processing system 250. A splitter 216 splits the modulatedoptical waveform into a transmitted signal 205 with most of the energyof the beam 203 and a reference signal 207 with a much smaller amount ofenergy that is nonetheless enough to produce good heterodyne or homodyneinterference with the returned light 291 scattered from a target (notshown). In some embodiments, the transmitted beam is scanned overmultiple angles to profile any object in its path using scanning optics218.

The reference beam is delayed in a reference path 220 sufficiently toarrive at the detector array 230 with the scattered light. In someembodiments, the splitter 216 is upstream of the modulator 214, and thereference beam 207 is unmodulated. In some embodiments, the referencesignal is independently generated using a new laser (not shown) andseparately modulated using a separate modulator (not shown) in thereference path 220 and the RF waveform from generator 215. In someembodiments, as described below with reference to FIG. 2B, a differentmodulator is used; but, only one laser source 212 is used for bothtransmitted and reference signals to ensure coherence. In variousembodiments, from less to more flexible approaches, the reference iscaused to arrive with the scattered or reflected field by: 1) putting amirror in the scene to reflect a portion of the transmit beam back atthe detector array so that path lengths are well matched; 2) using afiber delay to closely match the path length and broadcast the referencebeam with optics near the detector array, as suggested in FIG. 2A, withor without a path length adjustment to compensate for the phasedifference observed or expected for a particular range; or, 3) using afrequency shifting device (acousto-optic modulator) or time delay of alocal oscillator waveform modulation to produce a separate modulation tocompensate for path length mismatch; or some combination. In someembodiments, the target is close enough and the pulse duration longenough that the returns sufficiently overlap the reference signalwithout a delay. In some embodiments, the reference signal 207 b isoptically mixed with the return signal 291 at one or more optical mixers232.

In various embodiments, multiple portions of the target scatter arespective returned light 291 signal back to the detector array 230 foreach scanned beam resulting in a point cloud based on the multipleranges of the respective multiple portions of the target illuminated bymultiple beams and multiple returns. The detector array is a single orbalanced pair optical detector or a 1D or 2D array of such opticaldetectors arranged in a plane roughly perpendicular to returned beams291 from the target. The phase or amplitude of the interface pattern, orsome combination, is recorded by acquisition system 240 for eachdetector at multiple times during the pulse duration τ. The number oftemporal samples per pulse duration affects the down-range extent. Thenumber is often a practical consideration chosen based on pulserepetition rate and available camera frame rate. The frame rate is thesampling bandwidth, often called “digitizer frequency.” Basically, if Xnumber of detector array frames are collected during a pulse withresolution bins of Y range width, then a X*Y range extent can beobserved. The acquired data is made available to a processing system250, such as a computer system described below with reference to FIG. 8, or a chip set described below with reference to FIG. 9 . In someembodiments, the acquired data is a point cloud based on the multipleranges of the respective multiple portions of the target.

A Doppler compensation module 270 determines the size of the Dopplershift and the corrected range based thereon. In some embodiments, theDoppler compensation module 270 controls the RF waveform generator 215.

FIG. 2B depicts an alternative hardware arrangement that allows softwarecontrolled delays to be introduced into the reference path that producesthe LO signal. The laser source 212, splitter 216, transmit signal 205,scanning optics 218, optical mixers 232, detector array 230, acquisitionsystem 240 and processing system 250 are as described above withreference to FIG. 2A. In FIG. 2B, there are two separate opticalmodulators, 214 a in the transmit path and 214 b in the reference pathto impose the RF waveform from generator 215 onto an optical carrier.The splitter 216 is moved between the laser source 212 and themodulators 214 a and 214 b to produce optical signal 283 that impingeson modulator 214 a and lower amplitude reference path signal 287 a thatimpinges on modulator 214 b in a revised reference path 282. In thisembodiment, the light 201 is split into a transmit (TX) path beam 283and reference/local oscillator (LO) path beam 287 a before themodulation occurs; and, separate modulators are used in each path. Withthe dual modulator approach, either path can be programmed with chirpsat offset starting frequencies and/or offset starting times. This can beused to allow the system 280 to garner range delay effects for chirpDoppler compensation. By shifting the delay used in each range gate, thesystem can unambiguously measure with high resolution despite othersystems limitations (detector and digitizer bandwidth, measurement time,etc.). Thus, in some embodiments, a revised Doppler compensation module278 controls the RF waveform generator 215 to impose the delay timeappropriate for each Doppler shift produced by the Doppler compensationmodule described below. The software controlled delay reference signal287 b is then mixed with the return signals 291, as described above. Inother embodiments, the software controlled delay of the LO referencepath 282 allows an adaptive scanning approach to be adaptive in thedown-range dimension as well.

For example, in some embodiments, the laser used was actively linearizedwith the modulation applied to the current driving the laser.Experiments were also performed with electro-optic modulators providingthe modulation. The system is configured to produce a chirp of bandwidthB and duration r, suitable for the down-range resolution desired, asdescribed in more detail below for various embodiments. For example, insome illustrated embodiments, a value for B of about 90 GHz and τ ofabout 200 milliseconds (ms, 1 ms=10 seconds) were chosen to work withinthe confines of the relatively low detector array frame rate in theexperiments performed. These choices were made to observe a reasonablylarge range window of about 30 cm, which is often important indetermining a shape of an object and identification of the object. Thistechnique will work for chirp bandwidths from 10 MHz to 5 THz. However,for the 3D imaging applications, typical ranges are chirp bandwidthsfrom about 300 MHz to about 20 GHz, chirp durations from about 250nanoseconds (ns, ns=10⁻⁹ seconds) to about 1 millisecond (ms, 1 ms=10⁻³seconds), ranges to targets from about 0 meters to about 20 km, spotsizes at target from about 3 millimeters (mm, 1 mm=10−3 meters) to about1 meter (m), depth resolutions at target from about 7.5 mm to about 0.5m. It is noted that the range window can be made to extend to severalkilometers under these conditions and that the Doppler resolution canalso be quite high (depending on the duration of the chirp). Althoughprocesses, equipment, and data structures are depicted in FIG. 2 asintegral blocks in a particular arrangement for purposes ofillustration, in other embodiments one or more processes or datastructures, or portions thereof, are arranged in a different manner, onthe same or different hosts, in one or more databases, or are omitted,or one or more different processes or data structures are included onthe same or different hosts. For example splitter 216 and reference path220 include zero or more optical couplers.

FIG. 3A is a block diagram that illustrates example components of aheterodyne chirped LIDAR system 300 a, according to an embodiment. Thissystem 300 a, modified from that described in U.S. Pat. No. 7,742,152,uses electronic de-chirping. Although an object 390 is depicted toillustrate operation of the system 300 a, the object 390 is not part ofthe system 300 a. System 300 a includes laser 301, modulator 310,telescope as a scanning optical coupler 320, balanced photodetector 330,processing circuitry 340, waveform generator 350 that produces an FMchirp of bandwidth B and duration r, power splitter 351, de-chirpingmixer 360, and acoustic-optic modulator 370. In this system, the sourcelaser 301 output beam is split by beam splitter 302 into two parts; onepart is modulated by modulator 310 based on the FM chirp from powersplitter 351 and operational amplifier 352 a to produce beam 305 that isfed to the telescope.

The other part of the beam, beam 307 a is used to generate a localoscillator (LO) for coherent detection. An acoustic speaker produces anacoustic signal with frequency ƒm to drive an acousto-optic modulator(AOM) 370 to shift the optical frequency by ƒm in beam 307 b, whichserves as an intermediate frequency (IF) for heterodyne detection.Optical coupler 322 directs beam 307 b onto one of the balancedphotodetector 330.

A return optical signal 391 is also directed by optical coupler 322 tothe other part of the balanced photodetector. The balanced photodiode330 rejects the direct detection component. The output electrical signalis amplified in operational amplifier 344 a and the IF signal isselected by a bandpass filter 341 and detected by a Schottky diode 342which recovers the baseband waveform. The resulting electrical signal isdirected through low pass filter 343 and operational amplifier 344 b.

A de-chirping mixer compares this detected signal with the originalchirp waveform output by power splitter 351 and operational amplifier352 b to produce an electrical signal with the beat frequency thatdepends on the frequency difference between the RF reference waveformand the detected waveform. Another operational amplifier 344 c and a FFTprocess 345 is used to find the beating frequency. Processor 346 isprogrammed to do data analysis. Coherent detection systems like 300 asignificantly improve receiver signal to noise ratio (SNR) compared todirect detection of pulse travel time, however, at the cost of greatlyincreased system complexity. The electrical components from operationalamplifier 344 a and de-chirping mixer 360 through processor 346constitute a signal processing component 340.

According to the illustrated embodiment, the light beam emitted fromoptical coupler 320 impinges on one or more objects 390 with a finitebeam size that illuminates an illuminated portion 392 of the one or moreobjects. Backscattered light from an illuminated portion is returnedthrough the telescope to be directed by optical coupler 322 onto theoptical detector, such as one photodiode of a balanced photodetector330. The one or more objects are moving relative to the system 300 awith object velocity components 394, which can differ for returns fromdifferent parts of the illuminated portion. Based on this motion, thefrequency of a signal detected by system 300 a differs from thefrequency based solely on the range. The processor 346 includes aDoppler compensation module 380, as described below, to detect theDoppler effect, and to correct the resulting range for the Dopplereffect.

FIG. 3B is a block diagram that illustrates example components of ahomodyne chirped LIDAR system 300 b, according to an embodiment. Thissystem 300 b, modified from that described in U.S. Pat. No. 7,742,152,uses photonic de-chirping and simplifies the RF components. Although anobject 390 is depicted to illustrate operation of the system 300 b, theobject 390 is not part of the system 300 b. The system 300 b includeswaveform generator 350, laser 301, modulator 310, splitter 302downstream of the modulator 310, telescope used as scanning opticalcoupler 320, balanced photodetector 330, and processing circuitry 360.

In this system, both the optical signal and the local oscillator LO aredriven by the same waveform generator 350 and amplified in operationalamplifier 352. The beam output by the modulator 310 is split by beamsplitter 302 to a beam part 305 and a beam part 307 c. The beam part305, with most of the beam energy, e.g., 90% or more, is transmittedthrough the optical coupler 320 to illuminate the illuminated portion392 of the object 390 moving with velocity component 394, as describedabove. The beam part 307 c is delayed a desired amount in delay 308 toproduce the reference signal 307 d. In some embodiments, there is nodelay and delay 308 is omitted. The reference signal 307 d and thereturn signal 309 from the telescope or other optical coupler 320 aredirected to the photodetector 330 by optical couplers 322. In someembodiments, described in more detail below, a frequency shifter 318 isadded to the optical path of the transmitted signal 305 beforetransmission through the coupler 320. In other embodiments describedbelow, the frequency shifter 318 is disposed in the optical path of thereturn signal 309 after passing through the coupler 320.

The de-chirping process is accomplished within the balanced photodiode330 and therefore eliminates the need of de-chirping mixing and theassociated RF processing. Because the original chirp optical waveform,which is carried by the LO, beats with its delayed version at thephotodiode as indicated, target distance can be directly obtained by afrequency analysis in an FFT component 345 of the photocurrent signaloutput by operational amplifier 344. Processor 362 is programmed to dodata analysis. The processor 362 includes a Doppler compensation module380, as described below, to detect the Doppler effect, and to correctthe resulting range for the Doppler effect. The electrical componentsfrom operational amplifier 344 through processor 362 constitute a signalprocessing component 360. Considering that shot noise is the dominantnoise with coherent detection, SNR at the beating frequency is reducedcompared to SNR of direct detection and SNR of the system 300 a.

3. Doppler Effect Detection Overview

FIG. 4A is a block diagram that illustrates an example graph 410 showingrange, based on beat frequency peaks from an up chirp LIDAR system, toseveral objects when the objects are moving, according to an embodiment.The horizontal axis 412 indicates range in arbitrary units based on beatfrequency, and the vertical axis 414 indicates peak power in relativeunits. Because the chirp is an up chirp (higher frequencies at latertimes) an increase in frequency is translated into an increase in traveltime and therefore an increase in range. A beat frequency is plotted asan arrow located at the range associated with the beat frequency with aheight that indicates relative power of the peak. Three observed peaks414 a, 414 b and 414 c are indicated by solid arrows at the respectiveranges. The last peak 414 c, due to error, will be discussed in moredetail below.

The first observed peak 414 a is due to a first scatterer moving towardthe LIDAR system at a speed sufficient to introduce a shift towardhigher frequencies, called a blue shift. The range effect 415 of theblue shift is depicted. Since the chirp is an up chirp, as depicted ingraph 120 of FIG. 1A, the increase in frequency is associated with anincreased range, and the actual position 413 a is to the left of theinferred range. The object is actually at a range that, if there were noDoppler effect, would be at a range 413 a. But neither the blue shiftrange effect 415 nor the actual range 413 a is known to the LIDAR systemdata processing component.

In contrast, the second observed peak 414 b is due to a second scatterermoving away from the LIDAR system at a speed sufficient to introduce ashift toward lower frequencies, called a red shift. The range effect 416of the red shift is depicted. Since the chirp is an up chirp, thedecrease in frequency is associated with a decreased range, and theactual position 413 b is to the right of the inferred range. The objectis actually at a range that, if there were no Doppler effect, would beat a range 413 b. But neither the red shift range effect 416 nor theactual range 413 b is known to the LIDAR system data processingcomponent.

One approach to determine the size of the Doppler effect on range is todetermine range once with an up chirp and again using a down chirp, inwhich the transmitted optical signal decreases in frequency with time.With a down chirp transmitted signal, the later arrivals have lowerfrequencies rather than higher frequencies and each red shift, or blueshift, would have the opposite effect on range than it does with an upchirp transmitted signal. The two oppositely affected ranges can becombined, e.g., by averaging, to determine a range that has reduced oreliminated Doppler effect. The difference between the two ranges canindicate the size of the Doppler effect (e.g., the range effect can bedetermined to be half the difference in the two ranges).

FIG. 4B is a block diagram that illustrates an example graph 420 showingrange, based on beat frequency peaks from a down chirp LIDAR system, toseveral objects when the objects are moving, according to an embodiment.The horizontal axis 412 and vertical axis 414 are as described above.Because the chirp is a down chirp (lower frequencies at later times) adecrease in frequency is translated into an increase in travel time andtherefore an increase in range. Only two observed peaks 424 a and 424 bare indicated by solid arrows at the respective ranges.

As stated above, the first observed peak 414 a is due to the firstscatterer moving toward the LIDAR system at a speed sufficient tointroduce a shift toward higher frequencies, called a blue shift. Therange effect 425 of the blue shift is different than for the up chirp;the range effect goes in the opposite direction. Since the chirp is adown chirp, the increase in frequency is associated with a decreasedrange, and the actual position 413 a, the same range as in graph 410, isto the right of the inferred range. As also stated above, the secondobserved peak 424 b is due to a second scatterer moving away from theLIDAR system at a speed sufficient to introduce a shift toward lowerfrequencies, called a red shift. The range effect 426 of the red shiftis toward longer ranges. The object is actually at a range 413 b to theleft, closer, than the inferred range.

As stated above, the two oppositely affected ranges can be combined,e.g., by averaging, to determine a range that has reduced or eliminatedDoppler effect. As can be seen, a combination of range 414 a with 424 a,such as an average, would give a value very close to the actual range413 a. Similarly, a combination of range 414 b with 424 b, such as anaverage, would give a value very close to the actual range 413 b. Themagnitude of the range effect 415 for object A (equal to the magnitudeof the range effect 425 for object A) can be determined to be half thedifference of the two ranges, e.g., ½*(range of 414 a-range of 425 a).Similarly, the magnitude of the range effect 416 for object B (equal tothe magnitude of the range effect 426 for object B) can be determined tobe half the difference of the two ranges, e.g., ½*(range of 424 b-rangeof 414 b). In some embodiments, the up-chirp/down-chirp returns arecombined in a way that incorporates range extent to theup-chirp/down-chirp peaks detected. Basically, the distributions of thedetected peak on the up-chirp/down-chirp return is combined (manydifferent statistical options) in an effort to reduce the variance ofthe Doppler estimate.

This correction depends on being able to pair a peak in the up chirpreturns with a corresponding peak in the down chirp returns. In somecircumstances this is obvious. However, the inventors have noted that,because of spurious peaks such as peak 414 c, it is not always clear howto automatically determine which peaks to pair, e.g., which of the threepeaks of FIG. 4A to pair with each of the two peaks of FIG. 4B. Inaddition, the number of spurious peaks due to clutter in the field ofview or other noise factors and the changes in range associated with amoving object and moving LIDAR system mean that the pairing of peaks ismade difficult when up chirp and down chirp transmitted pulses areseparated in time.

FIG. 5A is a graph that illustrates an example serial (also calledsequential) up and down chirp transmitted optical signal for a LIDARsystem, according to an embodiment. The horizontal axis 512 indicatestime in arbitrary units; and, the vertical axis 514 indicates frequencyof the optical transmitted signal or reference signal in arbitraryunits. A first pulse of duration 515 a is an up chirp and a second pulseof duration 515 b is a down chirp. The pairs of ranges obtained from theup chirp and down chirp can be combined to determine a better estimateof range and the Doppler effect. Any of the LIDAR systems in FIG. 2 ,FIG. 3A or FIG. 3B can be operated with such sequential up and downchirps. There is no requirement that the bandwidths or pulse duration,or both, be the same for both up chirp and down chirp pulses, as long aseach spans the range values of interest for a particular application.

As indicated above, if the sequential range measurements can besuccessfully paired and averaged, the range and Doppler of the targetcan be correctly inferred by averaging the range of the sequentialmeasurements. However, the sequential up/down approach leads to errorswhen the Doppler shift changes between measurements or when a translatedbeam (e.g., a LIDAR scanner) translates to a new location betweenmeasurements which could lead to a change in the objects being measuredor a change in the actual range to the same objects, or somecombination. As explained in more detail below, a cost function is usedto generate a cost matrix that is then used to determine a desirablepairing of ranges from up and down chirps.

In some embodiments, the LIDAR system is changed to produce simultaneousup and down chirps. This approach eliminates variability introduced byobject speed differences, or LIDAR position changes relative to theobject which actually does change the range, or transient scatterers inthe beam, among others, or some combination. The approach thenguarantees that the Doppler shifts and ranges measured on the up anddown chirps are indeed identical and can be most usefully combined. TheDoppler scheme guarantees parallel capture of asymmetrically shiftedreturn pairs in frequency space for a high probability of correctcompensation.

FIG. 5B is a graph that illustrates an example simultaneous up and downchirp transmitted optical signal for a LIDAR system, according to anembodiment. The horizontal axis indicates time in arbitrary units, notnecessarily the same axis limits as in FIG. 5A. For example, in someembodiments, the full temporal range of axis 524 is half the fulltemporal range of axis 512. The vertical axis 522 indicates frequency ofthe optical transmitted signal or reference signal in arbitrary units.During a pulse of duration 252 a, a light beam comprising two opticalfrequencies at any time is generated. One frequency increases, e.g.,from ƒ₁ to ƒ₂ while the other frequency simultaneous decreases from ƒ₄to ƒ₃. The two frequency bands e.g., band 1 from ƒ₁ to ƒ₂, and band 2from ƒ₃ to ƒ₄) do not overlap so that both transmitted and returnsignals can be optically separated by a high pass or a low pass filter,or some combination, with pass bands starting at pass frequency ƒ_(p).For example ƒ₁<ƒ₂<ƒ_(p)<ƒ₃<ƒ₄, Though, in the illustrated embodiment,the higher frequencies provide the down chirp and the lower frequenciesprovide the up chirp, in other embodiments, the higher frequenciesproduce the up chirp and the lower frequencies produce the down chirp.

In some embodiments, two different laser sources are used to produce thetwo different optical frequencies in each beam at each time. However, insome embodiments, a single optical carrier is modulated by a single RFchirp to produce symmetrical sidebands that serve as the simultaneous upand down chirps. In some of these embodiments, a double sidebandMach-Zehnder modulator is used that, in general, does not leave muchenergy in the carrier frequency; instead, almost all of the energy goesinto the sidebands.

FIG. 5C is a graph that illustrates example first order sidebandsproduced by a modulator to generate simultaneous up and down chirptransmitted optical signal for a LIDAR system, according to anembodiment. The horizontal axis 532 indicates frequency of a signal andthe vertical axis 534 indicates power of the signal. When an opticalcarrier ƒ₀ is modulated by a RF signal of frequency ƒ, two opticalsidebands are produced for each multiple of the RF frequency. The firstorder sidebands are at the RF frequency ƒ above and below the opticalcarrier frequency ƒ₀ indicated by arrows 536 a and 536 b. Second ordersidebands are produced at 2ƒ above and below the optical carrier ƒ₀,etc. Each higher order sideband is reduced in power compared to theprevious lower order sideband.

When producing an optical chirp using a RF down chirp varying from ƒb toƒa<ƒb, the bandwidth B=(ƒb−ƒa). The upper sideband varies fromƒ₀+ƒa+B=ƒ₀+ƒb to ƒ₀+ƒa, as indicated by the left pointing arrow onfrequency 536 a, producing a signal in band 538 a. The lower sidebandsimultaneous varies from ƒ₀−fa−B=ƒ₀−ƒb to ƒ₀−ƒa, as indicated by theright pointing arrow on frequency 536 b, producing a signal in band 538b. In other embodiments, a RF down chirp is used to modulate the opticalcarrier, and the frequencies 536 a and 536 b move through the bands 538a and 538 b, respectively, in the opposite directions, e.g., from leftto right in band 538 a and right to left in band 538 b. The returns fromthe up-chirp and the down chirp are distinguished using differentmethods in different embodiments. In some preferred embodiments theseparation is performed by adding a frequency shift to remove thesymmetry of the upper and lower sidebands, as described below. In otherembodiment, in which the sidebands are widely enough separated to beoptically filtered, the signals from each are split. One signal fromeach of the reference and return is passed through a low pass filterstarting at ƒ_(pl) to filter out the carrier ƒ₀ and the high band 538 ato obtain the low frequency band 538 b. Similarly, one signal from eachof the reference and return is passed through a high pass filterstarting at ƒ_(ph) to filter out the carrier ƒ₀ and the low band 538 bto obtain the high frequency band 538 a. The two bands are processed asdescribed above to produce the up-chirp ranges and the down-chirpranges. After pairing the ranges from the up chirp and down chirp, theDoppler effect and the corrected ranges are determined.

As a result of sideband symmetry, the bandwidth of the two opticalchirps will be the same if the same order sideband is used. In otherembodiments, other sidebands are used, e.g., two second order sidebandare used, or a first order sideband and a non-overlapping secondsideband is used, or some other combination.

FIG. 3B also illustrates example components for a simultaneous up anddown chirp LIDAR system according to one embodiment. In this embodiment,the frequency shifter 318 is added to the optical path of thetransmitted beam 305. In other embodiments, the frequency shifter isadded instead to the optical path of the returned beam 309 or to thereference path. In general, the frequency shifting element is added onthe local oscillator (LO, also called the reference path) side or on thetransmit side (before the optical amplifier) as the device (AOM) hassome loss associated and it is disadvantageous to put lossy componentson the receive side or after the optical amplifier. The purpose of theoptical shifter 318 is to shift the frequency of the transmitted signal(or return signal) relative to the frequency of the reference signal bya known amount Δfs, so that the beat frequencies of the up and downchirps occur in different frequency bands, which can be picked up, e.g.,by the FFT component 345, in the analysis of the electrical signaloutput by the optical detector. For example, if the blue shift causingrange effects 415 and 425 of FIG. 4A and FIG. 4B, respectively, isƒ_(B), then the beat frequency of the up chirp will be increased by theoffset and occur at ƒ_(B)+Δfs and the beat frequency of the down chirpwill be decreased by the offset to ƒ_(B)−Δfs. Thus, the up chirps willbe in a higher frequency band than the down chirps, thereby separatingthem. If Δfs is greater than any expected Doppler effect, there will beno ambiguity in the ranges associated with up chirps and down chirps.The measured beats can then be corrected with the correctly signed valueof the known Δfs to get the proper up-chirp and down-chirp ranges. Insome embodiments, the RF signal coming out of the balanced detector isdigitized directly with the bands being separated via FFT. In someembodiments, the RF signal coming out of the balanced detector ispre-processed with analog RF electronics to separate a low-band(corresponding to one of the up chirp or down chip) which can bedirectly digitized and a high-band (corresponding to the opposite chirp)which can be electronically down-mixed to baseband and then digitized.Both embodiments offer pathways that match the bands of the detectedsignals to available digitizer resources.

When selecting the transmit (TX) and local oscillator (LO) chirpwaveforms, it is advantageous to ensure that the frequency shifted bandsof the system take maximum advantage of available digitizer bandwidth.In general this is accomplished by shifting either the up chirp or thedown chirp to have a range frequency beat close to zero.

For example, in another embodiment, the transmitted (TX) signal and thereference (LO) signal are generated independently using upper and lowersidebands on two different modulators on the carrier frequency. FIG. 5Dis a graph that illustrates an example simultaneous up and down chirptransmitted optical signal and reference signal (LO) for a LIDAR systemrelative to an optical carrier, according to another embodiment. Thehorizontal axis is time in relative units, and the vertical axis isfrequency relative to the optical carrier ƒ₀ plotted as ƒ₀=0. Thereference signal (LO) is offset from the transmitted signal (TX) by anoffset frequency ƒ₀ that, because of the side band symmetries, andunlike the examples above, are on opposite sides on the up chirpcompared to the down chirp. In the illustrated embodiment, the LO signalfrequency is lower than the simultaneous TX signal frequency for the upchirp and the LO signal frequency is higher than the simultaneous TXsignal frequency for the down chirp. Thus, the TX up-chirp waveformchirps over bandwidth B from ƒ to ƒ+B; and the LO up-chirp waveformchirps from ƒ−ƒo to ƒ−ƒo+B. The down chirps are reflected around theoptical carrier frequency plotted as 0.

In this case, to get an up-chirp beat frequency near zero, it makessense to select the shift frequency Δƒs=ƒo such that the up chirp isaligned with the transmit. The down chirps will be separated by 2*ƒo.FIG. 5E is a graph that illustrates an example shifted simultaneous upand down chirp transmitted optical signal and reference signal (LO) fora LIDAR system relative to an optical carrier, according to anotherembodiment. The horizontal axis is time in relative units, and thevertical axis is frequency relative to the optical carrier ƒ₀ plotted asƒ₀=0. Overall this gives rise to an up-chirp beat frequency band forƒ_(R) from 0 to 2*ƒo and a non-overlapping down-chirp beat frequencyband for ƒ_(R) from 2*ƒo to a system cutoff frequency (probablydigitizer limited). If the down chirp beat frequency is too large, adown-mix stage for the large beat frequency band would mix the largebeat frequency band down by 2*ƒo so that it can be digitized atbase-band.

Any frequency shifter 318 known in the art may be used. For example, insome embodiments an acousto-optic modulator (AOM) is used; and, in otherembodiments, serradyne based phase shifting is used with a phasemodulator.

FIG. 6 is a block diagram that illustrates example components for asimultaneous up and down chirp LIDAR system 600, according to anotherembodiment. Although an object 390 is depicted to illustrate operationof the system 600, the object 390 is not part of the system 600. System600 is modified from system 300 b in FIG. 3B by adding a second detectorand processing for the simultaneous second chirp reference signal andreturn signal, as described in more detail below. In some embodiments,system 600 is modified from system 300 b in FIG. 3B by also adding asecond laser 601, second waveform generator 650 and operationalamplifier 652 and modulator 610 to generate the second simultaneouschirp signal. However, in some embodiments in which a single modulatorwith multiple sidebands is used to produce simultaneous up and downchirps, the second laser and components are omitted.

For example, in some embodiments, the laser 601 produces an opticalcarrier at frequency ƒ₀, the waveform generator 650 produces an RF downchirp from ƒ_(b) to ƒ_(a), where ƒ_(a)<ƒ_(b), with power augmented byoperational amplifier 652, to drive modulator 610 to produce the opticalcarrier frequency ƒ₀ and optical sidebands 538 a and 538 b depicted inFIG. 5C. This provides the simultaneous up and downs chirps depicted inFIG. 5B, where: ƒ₁=(ƒ₀−ƒ_(b)); ƒ₂=(ƒ₀−ƒ_(a)); ƒ₃=(ƒ₀+ƒ_(a)); andƒ₄=(ƒ₀+ƒ_(b)) and the up and down chirps are symmetrical. In theseembodiments, the laser 301, waveform generator 350, operationalamplifier 352 and modulator 310 are omitted.

In either type of embodiments, the optical signal entering beam splitter602 includes the simultaneous up chirp and down chirp. This beam issplit into two beams of identical waveforms but different power, withmost of the power, e.g., 90% or more, passing as beam 605 through ascanning optical coupler 320 to impinge on an object 390. The remainingpower passes as beam 607 a to a delay 308 to add any desired delay orphase or other properties to produce reference signal 607 b. In someembodiments, delay 308 is omitted and reference signal 607 b isidentical to signal 607 a.

Optical coupler 624 is configured to split the reference signal into tworeference beams, and to split the returned signal received from theobject 390 through scanning optical coupler 320 into two returnedsignals. The beams can be split into equal or unequal power beams. Onereference signal and one returned signal pass through low pass filter626 to remove the optical carrier and high frequency band. The lowpassed reference and returned signals are directed onto photodetector330 to produce a beat electrical signal that is amplified in operationalamplifier 344 and transformed in FFT component 345 and fed to dataanalysis module 662. The other reference signal and the other returnedsignal pass through high pass filter 628 to remove the optical carrierand low frequency band. The high passed reference and returned signalsare directed onto photodetector 630 to produce a beat electrical signalthat is amplified in operational amplifier 644 a and transformed in FFTcomponent 645 and fed to data analysis module 662. The Dopplercompensation module 680 matches pairs of the ranges in the up chirp anddown chirp paths of system 600, determines the Doppler effects, orcorrects the ranges based on a corresponding Doppler effects, or both.In some embodiments, the module 680 also operates a device based on theDoppler effect or Doppler corrected range or some combination. Forexample, a display device 1214 is operated to produce an image or pointcloud of ranges to objects or speed of objects or some combination. Thecomponents 344, 345, 644, 645, 662 and 680 constitute a data processingsystem 660. In many embodiments, the separation of the up and downchirps in the sidebands is less than 1 GHz and not large enough to becleanly separated using existing optical filters. In such embodiments,the system of FIG. 3B, described, above is used instead of the system ofFIG. 6 .

As stated above, the third observed peak 414 c is an example extraneouspeak due to system error that can occur in an actual system in a naturalsetting and is not associated with a range to any actual object ofinterest. This can lead to problems in determining which up-chirp rangeand down-chirp range to pair when determining the Doppler effect orcorrected range or both. That is, a complication arises as FM chirpwaveform (FMCW) systems resolve returns from all scatterers along agiven line of site. In this scenario, any Doppler processing algorithmmust effectively deal with the requirement that multiple returns on theup-chirp and down-chirp range profile be paired correctly forcompensation. In the case of a single range return, one peak on the “up”and one peak on the “down” would be isolated and correctly paired. Theconfounding situation arises when multiple peaks exist, from multiplescatterers, on the “up” and “down” side. Which peaks should be pairedwith which to get the desired compensation effect has to be determined,and preferably is determined using a method that can be implemented toproceed automatically, e.g., on a processor or integrated circuit.

Here is demonstrated an approach to automatically pair up-chirp rangesand down-chirp ranges for calculating Doppler effects and Dopplercorrected ranges. This approach uses a bi-partite graph matchingformulation to achieve correct up/down return parings with highprobability. FIG. 7A is a block diagram that illustrates example pairsof up-chirp and down-chirp ranges, according to an embodiment. Each pairincludes one up-chirp range and one down-chirp range. This diagramassumes there are two down-chirp ranges, and three up-chip ranges, forexample as diagrammed in FIG. 4B and FIG. 4A, respectively, asdown-chirp ranges 424 a and 424 b, and up-chirp ranges 414 a, 414 b and414 c. There are six possibilities in paring the two down-chirp rangeswith the three up-chirp ranges, represented by the lines connectingdown-chirp ranges 1 and 2 with each of the three up-chirp ranges 1, 2and 3.

For each possible pair, a cost is determined. A cost is a measure ofdissimilarity between the two ranges in the pair. Any method may be usedto evaluate the dissimilarity. For example, in some embodiments, thecost for each pair is a difference in detected ranges for the tworanges. In some embodiments, the cost is a difference in detected peakheights associated with the two ranges. In some embodiments, the cost isa difference in peak widths associated with the two ranges. In someembodiments, the cost is a scalar valued function of two or more ofindividual dissimilarities, such as range difference, peak heightdifference, and peak width difference, among others. In someembodiments, the cost is a vector function with two or more elementsselected from range difference, peak height difference, and peak widthdifference, among others. The cost is represented by the symbol C_(nm),where m indicates an index for a down-chirp range and n represents anindex for an up-chirp range. FIG. 7B is a block diagram that illustratesan example cost matrix with elements corresponding to the example pairsof up chirp and down-chirp ranges depicted in FIG. 7A, according to anembodiment. In FIG. 7 , the indices are reversed, showing the down-chirprange index first, C_(mn).

The set (S_(up)) of range returns R_(i up) from the up-chirp rangeprofiles and the set (S_(down)) of range returns R_(j down) from thedown-chirp range profiles are determined using Equation 1b andfrequencies of peaks selected via a standard thresholding and peakfitting procedure (e.g., peak finding based on height and width of thepeak) of the FFT spectrum of the electrical output of thephotodetectors.S _(up) =[R _(1up) ,R _(2up) , . . . R _(Nup)]  (3a)andS _(down) =[R _(1down) ,R _(2down) , . . . R _(Mdown)]  (3b)with N “up” peaks and M “down” peaks. The two sets of range profilesthen are used to define a cost matrix C where each matrix element is afunction of the pair C_(nm)=F(R_(n,up), R_(m,down)). A good costfunction for static scenes (imager not moving relative to scene) issimply the magnitude of the Doppler effect for the pairing. F(R_(n,up),R_(m,down))=|R_(n,up), −R_(m,down)|/2 where |.| is the absolute valueoperation. The general approach allows for flexible definition of costfunctions. For example, the cost could include other parameters such asthe intensity of the set of peaks, the uncompensated range, externalinformation such as imager motion in the imaging space (for mobilescanning), or combinations thereof.

Once the cost matrix C is defined, the approach proceeds with abipartite graph matching procedure. This procedure often restricts thepairings so that a given up-chirp range can only be paired with a singledown-chirp range, and vice versa. Other constraints can be imposed, suchas that lines connecting matched pairs do not cross. In someembodiments, the matching process proceeds in a greedy manner wherebythe algorithm always chooses the lowest cost pairing, minimum (C_(mn)),from the set until no further pairings exist. Alternatively, theHungarian bi-partite graph matching algorithm (e.g., see Munkres, 1957)can be used to generate the optimal “lowest cost” set of pairingsaveraged over all pairs. In other embodiments, other graph matchingmethods are used. It has been found that with observed real worldscenarios, the greedy approach is both faster and sufficient for theintended purposes. This method was chosen as it limits a peak on eitherside to be paired with maximum of one peak on the other side. This isthe most “physical” interpretation of the pairing procedure as a peak onone side being paired with two on the other side would imply anon-physical or confounding scenario. There are certainly other matchingprocedures (that were looked at but did not perform as well in theexperimental embodiments). For example, a “range ordered” approachsorted the up and down returns according to range and sequentiallypaired them (from closest to furthest). Extra peaks on either side whenone side “ran out of peaks” were discarded. This failed in the commonscenario of close peaks with slight Doppler shifts. Similarly “amplitudeordered” sorted the peaks according to amplitude and paired them indescending order of amplitude. This method did not work well becausespeckle effects in coherent detection cause large variance in theamplitude of detected peaks. This led to many incorrect pairings. Thecost matrix approach seems to be the most general way of considering theoptions and minimizing globally across the set of options. In FIG. 7A,two low cost pairs were determined, represented by the solid lines. Thusone up-chirp range (3) is not paired with either of the two down-chirpranges.

FIG. 8 is a flow chart that illustrates an example method for using anup and down chirp LIDAR system to compensate for Doppler effects onranges, according to an embodiment. Although steps are depicted in FIG.8 as integral steps in a particular order for purposes of illustration,in other embodiments, one or more steps, or portions thereof, areperformed in a different order, or overlapping in time, in series or inparallel, or are omitted, or one or more additional steps are added, orthe method is changed in some combination of ways.

In step 801, a transceiver, e.g., a LIDAR system, is configured totransmit up and down chirped optical signals. A portion (e.g., 1% to10%) of each of the up chirp and down chirp signals is also directed toa reference optical path. The transceiver is also configured to receivea backscattered optical signal from any external object illuminated bythe transmitted signals. In other embodiments the up chirp and downchirp signals are transmitted simultaneously. In some embodiments, step801 includes configuring other optical components in hardware to providethe functions of one or more of the following steps as well, asillustrated for example in FIG. 3A or FIG. 3B or FIG. 6 , orequivalents. Note that the transmitted signal need not be a beam. Adiverging signal will certainly see a lot of different ranges andDoppler values within a single range profile; but, provide no crossrange resolution within an illuminated spot. However, it is advantageousto use a narrow beam which provides inherent sparsity that comes withpoint by point scanning to provide the cross range resolution useful toidentify an object.

In some embodiments, the up chirp optical signal is symmetric with thedown chirp, that is, they have the same bandwidth, B, and they have thesame duration, τ. But a range can be determined for any bandwidth andduration that spans the returns of interest. Thus, in some embodiments,the up chirp optical signals and downs chirp optical signals havedifferent bandwidths, Bu and Bd, respectively, or different durations,τu and τd, respectively, or some combination. In some embodiments, it isconvenient to produce slightly different values for B and τ, e.g., byusing one first order sideband and one second order sideband of anoptical carrier, rather than positive and negative first ordersidebands, as depicted in FIG. 5C. This can be done but it would have tobe accounted for in the Doppler processing. In the example embodimentsdescribed below, the chirps come out of the same modulator with the samebandwidth and timing—exactly. This guarantees that the Doppler and rangethe system observes perfectly overlap. Different B and τ would be usedin the case of a system that is directly modulating the laser frequency(versus using external electronic modulation). In that case two lasersor a sequential approach is useful for generating the up-chirp anddown-chirp, as depicted in FIG. 6 . This opens the door to different Band τ for the two lasers or the sequential chirp.

In step 802 the transmitted signal is directed to a spot in a scenewhere there might be, or might not be, an object or a part of an object.

In step 803, the returned up chirp return signal is separated from thedown chirp return signal. For example, the up chirp and down chirp arein different optical frequency bands and the separation is accomplishedby a return path that includes splitting the return signal into twocopies and passing each through a different optical filter. One filter(e.g., optical filter 626) passes the frequencies of the up chirp whileblocking the frequencies of the down chirp; and, the other filter (e.g.,optical filter 628) passes the frequencies of the down chirp whileblocking the frequencies of the up chirp. When the up chirp and downchirp are transmitted sequentially, the separation is done by processingthe signals in different time windows rather than by passing through anoptical filter, and both chirps can use the same frequency band.

In step 805, the separated up chirp return is combined with the up chirpreference signal at a first detector to determine zero or more frequencydifferences (or resulting beat frequencies) in the up chirp returnsignal. For example, the electrical signal from a detector is operatedon by a FFT module in hardware or software on a programmable processor.When the up chirp signal and down chirp signal are transmittedsimultaneously, then the up-chirp and down chirp portions of the returnsignal are separated using one or more of the methods and systemsdescribed above, e.g., a frequency shift of transmitted or return signalrelative to the reference signal as in FIG. 3B, or optical filtering asin FIG. 6 .

In step 807, one or more up-chirp ranges, R_(1up), . . . R_(Nup), aredetermined based on the one or more up chirp frequency differences (beatfrequencies), e.g., using Equation 1b or equivalent in processor 662. Inaddition, one or more down-chirp ranges, R_(1down), . . . R_(Mdown), aredetermined based on the one or more down chirp frequency differences(beat frequencies), e.g., using Equation 1b or equivalent in processor662. Returns that have no up chirp beat frequencies or have no downchirp beat frequencies are discarded during step 807. As a result ofstep 807, the sets S_(up) and S_(down) are generated, as expressed inEquations 3a and 3b for each transmitted signal illuminating some spotin a scene.

In step 811, values for elements of a cost matrix are determined, usingany of the measures of dissimilarity identified above, such asdifference in range, differences in peak height, differences in peakwidth, or differences in any other beat frequency peak characteristic,or some scalar or vector combination. In some embodiments, the cost foreach pair of ranges is a scalar weighted function of several of thesemeasures, e.g., giving highest weight (e.g., about 60%) to differencesin range, a moderate weight (e.g., about 30%) to differences in peakheight, and a low weight (e.g., about 10%) to differences in peak width.In some embodiments, a bias term is added to each element of the costmatrix, which accounts for the motion of the imaging system and thedirection in which it may be pointed. This is advantageous inembodiments for mobile imaging where a non-zero, signed Doppler valuemay be expected in the case that the imaging system is targeting astationary object but itself is in motion. In other embodiments, theobject is moving or the object and imaging are both moving and it isadvantageous to add bias terms based on the relative motion and trackingof the object. The magnitude of the correction depends only on theelement of the velocity on a line connecting the imaging system and thetarget, called the radial direction. Thus, in some embodiments, the biasdepends on the direction of beam pointing relative to the motion of thesensor. Note that the imaging system could itself be used to estimaterelative velocities. The cost matrix includes a cost for every pair ofranges in the two sets, one up-chirp range and one down-chirp range.

In step 813, at least one up-chirp range is matched to at least onedown-chirp range based on the cost matrix, for example using bi-partitegraph matching algorithms. It was found that greedy matching providesthe advantages of being simpler to implement, faster to operate, andsufficient to make good automatic matches for Doppler correctionpurposes. This approach has generated imagery with better Dopplercompensation than naïve approaches such as sequential pairing ofbrightest peaks on the up and down sides or always pairing of theclosest peaks in range and proceeding outwards, as shown, for example,in FIG. 9B and FIG. 9C, described below.

In step 815, at least one matched pair for the current illuminated spotis used to determine a Doppler effect at the spot for at least oneobject or portion of an object illuminated in the spot by differencingthe ranges in the pair. In some embodiments, the Doppler corrected rangeis determined based on combining the two ranges in the pair, e.g., byaveraging. In some embodiments, a weighted average is used if one of theup-chirp range or down-chirp range is considered more reliable. Onerange may be considered more reliable, for example, because it is basedon a broader bandwidth or longer duration. In some embodiments, ameasure of reliability is used to weight the range, giving more weightto the more reliable range. If several matched ranges are available forthe current illuminated spot, all matched pairs are used to findmultiple ranges or multiple Doppler effects for the spot.

In step 821 it is determined whether there is another spot to illuminatein a scene of interest, e.g., by scanning the scanning optical coupler320 to view a new spot in the scene of interest. If so, control passesback to step 802 and following steps to illuminate the next spot andprocess any returns. If not, then there are no further spots toilluminate before the results are used, and control passes to step 823.

In step 823, a device is operated based on the Doppler effect or thecorrected ranges. In some embodiments, this involves presenting on adisplay device an image that indicates a Doppler corrected position ofany object at a plurality of spots illuminated by the transmittedoptical signal. In some embodiments, this involves communicating, to thedevice, data that identifies at least one object based on a point cloudof Doppler corrected positions at a plurality of spots illuminated bytransmitted optical signal. In some embodiments, this involvespresenting on a display device an image that indicates a size of theDoppler effect at a plurality of spots illuminated by the transmittedoptical signal, whereby moving objects are distinguished from stationaryobjects and absent objects. In some embodiments, this involves moving avehicle to avoid a collision with an object, wherein a closing speedbetween the vehicle and the object is determined based on a size of theDoppler effect at a plurality of spots illuminated by the transmittedoptical signal. In some embodiments, this involves identifying thevehicle or identifying the object on the collision course based on apoint cloud of Doppler corrected positions at a plurality of spotsilluminated by the transmitted optical signal. Filtering the point clouddata based on Doppler has the effect of identifying and removingvegetation that may be moving in the breeze. Hard targets, man-madetargets, or dense targets are then better revealed by the filteringprocess. This can be advantageous in defense and surveillance scenarios.In the vehicle scenario—the Doppler can be used to segment targets (i.e.road surface versus moving vehicle).

4. Example Embodiments

In these example embodiments, the LIDAR system used componentsillustrated above to produce simultaneous up and down chirp transmittedsignals. This system is commercially available as the HRS-3D fromBLACKMORE SENSORS AND ANALYTICS, INC.™ of Bozeman Mont. In these exampleembodiments, greedy matching was used. The first match is the one withthe lowest cost. That pair is then removed and the pair with the nextlowest cost is used.

FIG. 9A through 9C are images that illustrates an example output on adisplay device based on Doppler corrected ranges and improvement overnon-corrected ranges, according to embodiments. The image of FIG. 9A wascollected using a wide field of view (FOV), three dimensional (3D) FMchirped waveform (FMCW) LIDAR system. This scene was imaged with B=2gigahertz (GHz, 1 GHz=10⁹ Hertz) optical bandwidth waveform. The darkpixels of the image represent no return, the white pixels representstable returns with negligible Doppler effect (up chirp-range anddown-chirp range are the same). The grey areas represent pixels with anon-negligible Doppler effect using a binary filter on Dopplermagnitude. The gray pixels demonstrate sensitivity to motion ofvegetation. The image clearly distinguishes vegetation from hardtargets. Post-processing eliminated unwanted intensity speckle on hardtarget surfaces. In the example embodiment, the threshold was pickedempirically. In other embodiments several thresholds ae selected withsome measure of goodness of result; and the threshold that gives thebest result is selected.

The image of FIG. 9B was collected using the same configuration as inFIG. 9A, but without performing Doppler compensation in the postdetector processing. The image of FIG. 9C was collected with the sameLIDAR system, but performing the Doppler correction using the method ofFIG. 8 . As can be seen the blurring caused by leaf motion in the treesis greatly reduced in FIG. 9C, allowing more individual branches andleaves to be discerned.

Another test was run with a fast frame rate, narrow field of view imagerthat produces 10,000 data-point point clouds per frame at a 10 Hz framerate. A test person ran back and forth in the field of view of thesensor. Each image of the person is cut from a time series of severalhundred 3D imaging frames (cut from the same 3D orientationperspective).

FIG. 10A is a set of three images that illustrate an example output on adisplay device based on Doppler corrected ranges for a targetapproaching a LIDAR system, according to an embodiment. Each dotindicates a perspective view in 3D space based on range to a spot on anobject, in this case a moving person. The dots are plotted with a greyscale that extends from light for no motion relative to LIDAR system toblack for a closing speed of 3.5 meters per second (m/s). White pixelsindicate no return. Three different images are superimposed. The image1012 clearly indicates a nearly stationary person on a nearly stationarysurface. The image 1013 clearly indicates the person with some movingparts and some almost stationary parts. The main body and forward legappear to be almost stationary, while the back leg and forward arm areboth clearly moving faster toward the LIDAR system, as would be expectedwith a walking gait. The image 1014 clearly indicates the person movingquickly. The speed of the body and forward leg is higher than the speedof the trailing leg, consistent with a running gait. The relativemovement is even more apparent in a color scale than it is in thegreyscale illustrated. Furthermore, the different speed parts of theperson are not displaced from their correct relative positions.

FIG. 10B is a set of three images that illustrate an example output on adisplay device based on Doppler corrected ranges for a target retreatingfrom a LIDAR system, according to an embodiment. Each dot indicates alocation in a perspective view in 3D space based on range to a spot onan object, in this case a moving person. The dots are plotted with agrey scale that extends from light for no motion relative to LIDARsystem to black for an opening speed of 3.5 m/s. White pixels indicateno return. Three different images are superimposed. The image 1022clearly indicates a person moving away at a moderate speed about −2 m/s.The image 1023 clearly indicates the person moving away at a greaterspeed about −3 m/s, with a plant leg somewhat slower consistent with arunning gait. The image 1024 clearly indicates the person moving away ata reduced speed about −2 m/s, with a plant leg and body somewhat slowerthan the raised leg, consistent with coming to a stop after a run. Therelative movement is even more apparent in a color scale than it is inthe greyscale illustrated. As above, the different speed parts of theperson are not displaced from their correct relative positions.

Thus the system not only correctly places all pixels on a person's formwith corrected ranges, but also ascertains the movement of the figurebased on the Doppler effect.

In another embodiment, the system is used to correct for swayingvegetation. A collimated laser beam is scanned through overhead trees.The probability of having multiple returns with Doppler values presentis very high, especially with a slight breeze or wind present. Suchvegetation provides dense scatterers separated closely in range along agiven line of sight. Wind itself can cause Doppler shifts in thevegetation, thus making the imaging of the vegetation even morechallenging. FIG. 11 is an image that illustrates an example output on adisplay device based on Doppler corrected ranges at high resolution,according to an embodiment. As this image demonstrates, very crispimagery can be obtained with the described technique. This providesstrong evidence that the technique is effective at providing veryrealistic imagery in such scenarios.

5. Computational Hardware Overview

FIG. 12 is a block diagram that illustrates a computer system 1200 uponwhich an embodiment of the invention may be implemented. Computer system1200 includes a communication mechanism such as a bus 1210 for passinginformation between other internal and external components of thecomputer system 1200. Information is represented as physical signals ofa measurable phenomenon, typically electric voltages, but including, inother embodiments, such phenomena as magnetic, electromagnetic,pressure, chemical, molecular atomic and quantum interactions. Forexample, north and south magnetic fields, or a zero and non-zeroelectric voltage, represent two states (0, 1) of a binary digit (bit)).Other phenomena can represent digits of a higher base. A superpositionof multiple simultaneous quantum states before measurement represents aquantum bit (qubit). A sequence of one or more digits constitutesdigital data that is used to represent a number or code for a character.In some embodiments, information called analog data is represented by anear continuum of measurable values within a particular range. Computersystem 1200, or a portion thereof, constitutes a means for performingone or more steps of one or more methods described herein.

A sequence of binary digits constitutes digital data that is used torepresent a number or code for a character. A bus 1210 includes manyparallel conductors of information so that information is transferredquickly among devices coupled to the bus 1210. One or more processors1202 for processing information are coupled with the bus 1210. Aprocessor 1202 performs a set of operations on information. The set ofoperations include bringing information in from the bus 1210 and placinginformation on the bus 1210. The set of operations also typicallyinclude comparing two or more units of information, shifting positionsof units of information, and combining two or more units of information,such as by addition or multiplication. A sequence of operations to beexecuted by the processor 1202 constitutes computer instructions.

Computer system 1200 also includes a memory 1204 coupled to bus 1210.The memory 1204, such as a random access memory (RAM) or other dynamicstorage device, stores information including computer instructions.Dynamic memory allows information stored therein to be changed by thecomputer system 1200. RAM allows a unit of information stored at alocation called a memory address to be stored and retrievedindependently of information at neighboring addresses. The memory 1204is also used by the processor 1202 to store temporary values duringexecution of computer instructions. The computer system 1200 alsoincludes a read only memory (ROM) 1206 or other static storage devicecoupled to the bus 1210 for storing static information, includinginstructions, that is not changed by the computer system 1200. Alsocoupled to bus 1210 is a non-volatile (persistent) storage device 1208,such as a magnetic disk or optical disk, for storing information,including instructions, that persists even when the computer system 1200is turned off or otherwise loses power.

Information, including instructions, is provided to the bus 1210 for useby the processor from an external input device 1212, such as a keyboardcontaining alphanumeric keys operated by a human user, or a sensor. Asensor detects conditions in its vicinity and transforms thosedetections into signals compatible with the signals used to representinformation in computer system 1200. Other external devices coupled tobus 1210, used primarily for interacting with humans, include a displaydevice 1214, such as a cathode ray tube (CRT) or a liquid crystaldisplay (LCD), for presenting images, and a pointing device 1216, suchas a mouse or a trackball or cursor direction keys, for controlling aposition of a small cursor image presented on the display 1214 andissuing commands associated with graphical elements presented on thedisplay 1214.

In the illustrated embodiment, special purpose hardware, such as anapplication specific integrated circuit (IC) 1220, is coupled to bus1210. The special purpose hardware is configured to perform operationsnot performed by processor 1202 quickly enough for special purposes.Examples of application specific ICs include graphics accelerator cardsfor generating images for display 1214, cryptographic boards forencrypting and decrypting messages sent over a network, speechrecognition, and interfaces to special external devices, such as roboticarms and medical scanning equipment that repeatedly perform some complexsequence of operations that are more efficiently implemented inhardware.

Computer system 1200 also includes one or more instances of acommunications interface 1270 coupled to bus 1210. Communicationinterface 1270 provides a two-way communication coupling to a variety ofexternal devices that operate with their own processors, such asprinters, scanners and external disks. In general the coupling is with anetwork link 1278 that is connected to a local network 1280 to which avariety of external devices with their own processors are connected. Forexample, communication interface 1270 may be a parallel port or a serialport or a universal serial bus (USB) port on a personal computer. Insome embodiments, communications interface 1270 is an integratedservices digital network (ISDN) card or a digital subscriber line (DSL)card or a telephone modem that provides an information communicationconnection to a corresponding type of telephone line. In someembodiments, a communication interface 1270 is a cable modem thatconverts signals on bus 1210 into signals for a communication connectionover a coaxial cable or into optical signals for a communicationconnection over a fiber optic cable. As another example, communicationsinterface 1270 may be a local area network (LAN) card to provide a datacommunication connection to a compatible LAN, such as Ethernet. Wirelesslinks may also be implemented. Carrier waves, such as acoustic waves andelectromagnetic waves, including radio, optical and infrared wavestravel through space without wires or cables. Signals include man-madevariations in amplitude, frequency, phase, polarization or otherphysical properties of carrier waves. For wireless links, thecommunications interface 1270 sends and receives electrical, acoustic orelectromagnetic signals, including infrared and optical signals, thatcarry information streams, such as digital data.

The term computer-readable medium is used herein to refer to any mediumthat participates in providing information to processor 1202, includinginstructions for execution. Such a medium may take many forms,including, but not limited to, non-volatile media, volatile media andtransmission media. Non-volatile media include, for example, optical ormagnetic disks, such as storage device 1208. Volatile media include, forexample, dynamic memory 1204. Transmission media include, for example,coaxial cables, copper wire, fiber optic cables, and waves that travelthrough space without wires or cables, such as acoustic waves andelectromagnetic waves, including radio, optical and infrared waves. Theterm computer-readable storage medium is used herein to refer to anymedium that participates in providing information to processor 1202,except for transmission media.

Common forms of computer-readable media include, for example, a floppydisk, a flexible disk, a hard disk, a magnetic tape, or any othermagnetic medium, a compact disk ROM (CD-ROM), a digital video disk (DVD)or any other optical medium, punch cards, paper tape, or any otherphysical medium with patterns of holes, a RAM, a programmable ROM(PROM), an erasable PROM (EPROM), a FLASH-EPROM, or any other memorychip or cartridge, a carrier wave, or any other medium from which acomputer can read. The term non-transitory computer-readable storagemedium is used herein to refer to any medium that participates inproviding information to processor 1202, except for carrier waves andother signals.

Logic encoded in one or more tangible media includes one or both ofprocessor instructions on a computer-readable storage media and specialpurpose hardware, such as ASIC 1220.

Network link 1278 typically provides information communication throughone or more networks to other devices that use or process theinformation. For example, network link 1278 may provide a connectionthrough local network 1280 to a host computer 1282 or to equipment 1284operated by an Internet Service Provider (ISP). ISP equipment 1284 inturn provides data communication services through the public, world-widepacket-switching communication network of networks now commonly referredto as the Internet 1290. A computer called a server 1292 connected tothe Internet provides a service in response to information received overthe Internet. For example, server 1292 provides information representingvideo data for presentation at display 1214.

The invention is related to the use of computer system 1200 forimplementing the techniques described herein. According to oneembodiment of the invention, those techniques are performed by computersystem 1200 in response to processor 1202 executing one or moresequences of one or more instructions contained in memory 1204. Suchinstructions, also called software and program code, may be read intomemory 1204 from another computer-readable medium such as storage device1208. Execution of the sequences of instructions contained in memory1204 causes processor 1202 to perform the method steps described herein.In alternative embodiments, hardware, such as application specificintegrated circuit 1220, may be used in place of or in combination withsoftware to implement the invention. Thus, embodiments of the inventionare not limited to any specific combination of hardware and software.

The signals transmitted over network link 1278 and other networksthrough communications interface 1270, carry information to and fromcomputer system 1200. Computer system 1200 can send and receiveinformation, including program code, through the networks 1280, 1290among others, through network link 1278 and communications interface1270. In an example using the Internet 1290, a server 1292 transmitsprogram code for a particular application, requested by a message sentfrom computer 1200, through Internet 1290, ISP equipment 1284, localnetwork 1280 and communications interface 1270. The received code may beexecuted by processor 1202 as it is received, or may be stored instorage device 1208 or other non-volatile storage for later execution,or both. In this manner, computer system 1200 may obtain applicationprogram code in the form of a signal on a carrier wave.

Various forms of computer readable media may be involved in carrying oneor more sequence of instructions or data or both to processor 1202 forexecution. For example, instructions and data may initially be carriedon a magnetic disk of a remote computer such as host 1282. The remotecomputer loads the instructions and data into its dynamic memory andsends the instructions and data over a telephone line using a modem. Amodem local to the computer system 1200 receives the instructions anddata on a telephone line and uses an infra-red transmitter to convertthe instructions and data to a signal on an infra-red a carrier waveserving as the network link 1278. An infrared detector serving ascommunications interface 1270 receives the instructions and data carriedin the infrared signal and places information representing theinstructions and data onto bus 1210. Bus 1210 carries the information tomemory 1204 from which processor 1202 retrieves and executes theinstructions using some of the data sent with the instructions. Theinstructions and data received in memory 1204 may optionally be storedon storage device 1208, either before or after execution by theprocessor 1202.

FIG. 13 illustrates a chip set 1300 upon which an embodiment of theinvention may be implemented. Chip set 1300 is programmed to perform oneor more steps of a method described herein and includes, for instance,the processor and memory components described with respect to FIG. 12incorporated in one or more physical packages (e.g., chips). By way ofexample, a physical package includes an arrangement of one or morematerials, components, and/or wires on a structural assembly (e.g., abaseboard) to provide one or more characteristics such as physicalstrength, conservation of size, and/or limitation of electricalinteraction. It is contemplated that in certain embodiments the chip setcan be implemented in a single chip. Chip set 1300, or a portionthereof, constitutes a means for performing one or more steps of amethod described herein.

In one embodiment, the chip set 1300 includes a communication mechanismsuch as a bus 1301 for passing information among the components of thechip set 1300. A processor 1303 has connectivity to the bus 1301 toexecute instructions and process information stored in, for example, amemory 1305. The processor 1303 may include one or more processing coreswith each core configured to perform independently. A multi-coreprocessor enables multiprocessing within a single physical package.Examples of a multi-core processor include two, four, eight, or greaternumbers of processing cores. Alternatively or in addition, the processor1303 may include one or more microprocessors configured in tandem viathe bus 1301 to enable independent execution of instructions,pipelining, and multithreading. The processor 1303 may also beaccompanied with one or more specialized components to perform certainprocessing functions and tasks such as one or more digital signalprocessors (DSP) 1307, or one or more application-specific integratedcircuits (ASIC) 1309. A DSP 1307 typically is configured to processreal-world signals (e.g., sound) in real time independently of theprocessor 1303. Similarly, an ASIC 1309 can be configured to performedspecialized functions not easily performed by a general purposedprocessor. Other specialized components to aid in performing theinventive functions described herein include one or more fieldprogrammable gate arrays (FPGA) (not shown), one or more controllers(not shown), or one or more other special-purpose computer chips.

The processor 1303 and accompanying components have connectivity to thememory 1305 via the bus 1301. The memory 1305 includes both dynamicmemory (e.g., RAM, magnetic disk, writable optical disk, etc.) andstatic memory (e.g., ROM, CD-ROM, etc.) for storing executableinstructions that when executed perform one or more steps of a methoddescribed herein. The memory 1305 also stores the data associated withor generated by the execution of one or more steps of the methodsdescribed herein.

6. Alterations, Extensions and Modifications

In the foregoing specification, the invention has been described withreference to specific embodiments thereof. It will, however, be evidentthat various modifications and changes may be made thereto withoutdeparting from the broader spirit and scope of the invention. Thespecification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense. Throughout thisspecification and the claims, unless the context requires otherwise, theword “comprise” and its variations, such as “comprises” and“comprising,” will be understood to imply the inclusion of a stateditem, element or step or group of items, elements or steps but not theexclusion of any other item, element or step or group of items, elementsor steps. Furthermore, the indefinite article “a” or “an” is meant toindicate one or more of the item, element or step modified by thearticle. As used herein, unless otherwise clear from the context, avalue is “about” another value if it is within a factor of two (twice orhalf) of the other value. While example ranges are given, unlessotherwise clear from the context, any contained ranges are also intendedin various embodiments. Thus, a range from 0 to 10 includes the range 1to 4 in some embodiments.

7. References

-   Adany, P., C. Allen, and R. Hui, “Chirped Lidar Using Simplified    Homodyne Detection,” Jour. Lightwave Tech., v. 27 (16), 15 Aug.    2009.-   Hui, R., C. Allen, and P. Adany, “Coherent detection scheme for FM    Chirped laser RADAR,” U.S. Pat. No. 7,742,152, 22 Jun. 2010.-   Kachelmyer, A. L., “Range-Doppler Imaging with a Laser Radar,” The    Lincoln Laboratory Journal, v. 3. (1), 1990.-   Munkres, J., “Algorithms for the Assignment and Transportation    Problems,” Journal of the Society for Industrial and Applied    Mathematics, v 5 (1), pp 32-38, March, 1957.

What is claimed is:
 1. A method for autonomous vehicle control usingobject range detection, comprising: transmitting a plurality of firstoptical signals that increase in frequency with time; receiving aplurality of first return signals from at least one of reflection orscattering of the plurality of first optical signals by an object;determining, by one or more processors, a plurality of first ranges toan object using the plurality of first return signals and a plurality offirst reference signals based on the plurality of first optical signals;transmitting a plurality of second optical signals that decrease infrequency with time; receiving a plurality of second return signals fromat least one of reflection or scattering of the plurality of secondoptical signals by the object; determining, by one or more processors, aplurality of second ranges to an object using the plurality of secondreturn signals and a plurality of second reference signals based on theplurality of second optical signals; determining, by the one or moreprocessors, a third range to the object by comparing, by the one or moreprocessors, each of the plurality of first ranges with each of theplurality of second ranges and selecting, by the one or more processors,a particular first range of the plurality of first ranges having a valuethat matches a particular second range of the plurality of second rangesresponsive to the comparison, wherein comparing, by the one or moreprocessors, each of the plurality of first ranges with each of theplurality of second ranges comprises determining, by the one or moreprocessors, a magnitude of a Doppler effect of a plurality of pairs ofeach of the plurality of first ranges and each of the plurality ofsecond ranges; and controlling operation of an autonomous vehicle basedat least on the third range to the object.
 2. The method of claim 1,further comprising determining the plurality of first ranges based onfrequency differences between the plurality of first reference signalsand the plurality of first return signals.
 3. The method of claim 1,further comprising transmitting the plurality of first optical signalswhile transmitting the plurality of second optical signals.
 4. Themethod of claim 1, wherein determining the third range comprisescorrecting for a Doppler effect associated with at least one of aparticular first return signal used to determine the particular firstrange or a particular second return signal used to determine theparticular second range.
 5. The method of claim 1, further comprisingtransmitting the plurality of first optical signals in a first frequencyband and transmitting the plurality of second optical signals in asecond frequency band that does not overlap the first frequency band. 6.The method of claim 1, further comprising presenting, using a displaydevice, a Doppler corrected position of the object.
 7. The method ofclaim 6, further comprising: determining that the object is a movingobject; and presenting, using the display device, the Doppler correctedposition to indicate that the object is a moving object.
 8. The methodof claim 1, further comprising: determining a closing speed between theautonomous vehicle and the object; and controlling the operation of theautonomous vehicle using the closing speed.
 9. The method of claim 1,further comprising: determining a shape of the object based at least onthe third range; and controlling the operation of the autonomous vehicleusing the shape of the object.
 10. The method of claim 1, whereincomparing, by the one or more processors, each of the plurality of firstranges with each of the plurality of second ranges comprisesdetermining, by the one or more processors, a cost matrix of a pluralityof pairs of each of the plurality of first ranges and each of theplurality of second ranges.
 11. The method of claim 1, whereincomparing, by the one or more processors, each of the plurality of firstranges with each of the plurality of second ranges comprisesdetermining, by the one or more processors, a characteristic of a beatfrequency peak of each of the plurality of first ranges and each of theplurality of second ranges.
 12. A light detection and ranging (LIDAR)system, comprising: a laser source configured to generate opticalsignals; a modulator configured to generate, using the optical signals,a plurality of first optical signals that increase in frequency withtime and a plurality of second optical signals that decrease infrequency with time; scanning optics configured to transmit theplurality of first optical signals and the plurality of second opticalsignals; and a processing circuit comprising one or more processorsconfigured to: determine a plurality of first ranges to an object usinga plurality of first return signals from at least one of reflection orscattering of the plurality of first optical signals by an object and atleast one first reference signal based on at least one first opticalsignal of the plurality of first optical signals; determine a pluralityof second ranges to an object using a plurality of second return signalsfrom at least one of reflection or scattering of the plurality of secondoptical signals by an object and at least one second reference signalbased on at least one second optical signal of the plurality of secondoptical signals; determine a third range to the object by selecting, bycomparing, by the one or more processors, each of the plurality of firstranges with each of the plurality of second ranges and selecting, by theone or more processors, a particular first range of the plurality offirst ranges having a value that matches a particular second range ofthe plurality of second ranges responsive to the comparison, whereincomparing, by the one or more processors, each of the plurality of firstranges with each of the plurality of second ranges comprisesdetermining, by the one or more processors, a magnitude of a Dopplereffect of a plurality of pairs of each of the plurality of first rangesand each of the plurality of second ranges; and output the third rangeto a vehicle controller configured to control operation of an autonomousvehicle based at least on the third range to the object.
 13. The LIDARsystem of claim 12, wherein the scanning optics comprise an opticalcoupler configured to transmit the plurality of first optical signalsand the plurality of second optical signals and receive the plurality offirst return signals and the plurality of second return signals.
 14. TheLIDAR system of claim 12, further comprising a detector array configuredto acquire the plurality of first return signals and the plurality ofsecond return signals.
 15. The LIDAR system of claim 12, wherein theprocessing circuit is configured to determine the particular first rangebased on frequency differences between a particular first referencesignal of the plurality of first reference signals and a particularfirst return signal of the plurality of first return signals.
 16. TheLIDAR system of claim 12, wherein the modulator is configured to outputthe plurality of first optical signals in a first frequency band and theplurality of second optical signals in a second frequency band that doesnot overlap the first frequency band.
 17. An autonomous vehicle system,comprising: a LIDAR sensor configured to: transmit a plurality of firstoptical signals that increase in frequency with time; receive aplurality of first return signals from at least one of reflection orscattering of the plurality of first optical signals by an object;determine, by one or more processors, a plurality of first ranges to anobject using the plurality of first return signals and at least onefirst reference signal based on at least one first optical signal of theplurality of first optical signals; transmit a plurality of secondoptical signals that decrease in frequency with time; receive aplurality of second return signals from at least one of reflection orscattering of the plurality of first optical signals by the object;determine, by the one or more processors, a plurality of second rangesto the object using the plurality of second return signals and at leastone second reference signal based on at least one second optical signalof the plurality of second optical signals; and determine, by the one ormore processors, a third range to the object by comparing, by the one ormore processors, each of the plurality of first ranges with each of theplurality of second ranges and identifying, by the one or moreprocessors, a particular first range of the plurality of first rangeshaving a value that matches a particular second range of the pluralityof second ranges responsive to the comparison, wherein comparing, by theone or more processors, each of the plurality of first ranges with eachof the plurality of second ranges comprises determining, by the one ormore processors, a cost matrix of a plurality of pairs of each of theplurality of first ranges and each of the plurality of second ranges;and a vehicle controller configured to control operation of anautonomous vehicle based at least on the third range to the object. 18.The autonomous vehicle system of claim 17, wherein the vehiclecontroller is configured to determine a shape of the object based atleast on the third range and control the operation of the autonomousvehicle using the shape of the object.
 19. The autonomous vehicle systemof claim 17, wherein the vehicle controller is configured to determine aclosing speed between the autonomous vehicle and the object and controlthe operation of the autonomous vehicle using the closing speed.